14 people were killed and around 45 others injured
A man from Central Asia is believed to have been behind two bombs on the St Petersburg Metro.
The first bomb was detonated on a train between Sennaya Square and Technological Institute, two busy stations in the centre of the Russian city on Monday afternoon.
At least 14 people were killed and dozens were injured in what is reported to have been a suicide attack, however, it is now not clear if the bomber got away.
Security services in Kyrgystan named the bomber as Akbarjon Djalilov, who was born in the country's second city of Osh and had Russian citizenship.
Russia's anti-terror agency said that less than two hours later, a second bomb was found and deactivated at Vosstaniya Square, a station that is a major transfer point for two Metro lines and also serves the rail line to Moscow.
That bomb, which was disguised as a fire extinguisher, was filled with shrapnel and more than three times the size of the first explosive.
The Interfax news agency cited an unidentified law enforcement official as saying that the suspected suicide bomber is believed to have left the larger bomb at Vosstaniya Square before blowing himself up on the train using the first bomb, which he carried in a backpack.
He is believed to have links to radical Islamist groups, although a spokesman for Russian President Vladimir Putin would not comment - saying it was up to law enforcement agencies to talk about the investigation.
Foreign Affairs Minister Charlie Flanagan said Monday he was "appalled" to hear of the explosion.
"I was deeply saddened and appalled to hear of the explosion that took place today on the St. Petersburg metro.
"I wish to offer the heartfelt condolences of the Irish people and the Government of Ireland to the families of those who have lost their lives and those who have been injured in the explosion.
"My thoughts and deepest sympathies are with the Russian people at this difficult time."
Meanwhile, the Kremlin said US President Donald Trump called Mr Putin to pass on his condolences following the attack.
In a statement, the Kremlin said Mr Trump had also asked Mr Putin to convey his support for the Russian people and that Mr Putin had thanked his American counterpart for the expression of solidarity.
A White House statement said that Mr Trump had "offered the full support of the US government in responding to the attack and bringing those responsible to justice".
Mr Putin was among those to visit an improvised memorial near the entrance to the Technological Institute station on Monday evening.
Earlier he told Russians that law enforcement and intelligence agencies were working to "give a full picture of what happened".
He promised help for victims of the explosion and their families.
The UN Security Council has also condemned "in the strongest terms the barbaric and cowardly terrorist attack" in Russia's second city.
Describing the moments after the explosion, student Maria Smirnova said: "Everything was covered in smoke, there were a lot of firefighters.
"Firefighters shouted at us to run for the exit and everyone ran. Everyone was panicking."
Investigator Svetlana Petrenko said the driver of the train saved lives by continuing on to the next stop, reducing the danger to passengers who would have had to walk along electrified tracks.
St Petersburg's metro has not been hit by terrorist attacks before but transport systems in other parts of Russia have, such as a suicide attack on a bus in Volgograd in 2013 which killed six people.
Also in 2013, 34 people died after twin bombings at Volgograd train station and on a bus in the city.